Clutches are used to provide a mechanical by-pass for the hydrodynamic coupling in torque converters in order to improve fuel economy in vehicles. The clutch is engaged and stays locked as soon as torque multiplication is no longer required. During clutch engagement, the engine vibration will be transferred via the clutch to the drivetrain which causes excess wear on components of the drivetrain as well as passenger discomfort. To reduce the transmitted torsional vibrations, torsional isolators or dampers are placed in clutches between the engine output torque and the transmission input shaft. Torsional dampers typically comprise an arrangement of springs and friction plates serving as an elastic member to reduce the engine vibration amplitude. To isolate today's engine with more and more power, many complex spring arrangements are being designed. However, they all can introduce extra undesirable resonant frequencies that cause or increase unwanted vibrations.
The problem of undesirable resonant frequencies is often addressed by using a plurality of series torsional isolators in torque converters. This trend is also driven by the push for improved fuel consumption and reduced lugging limits. To maintain or improve torsional isolation, lower spring rates are required. Cylinder shutoff applications and a general trend toward a decreased number of engine cylinders further enforce the need for lower spring rate (increased isolator spring volume). Due to envelope constraints, the most efficient way to increase spring volume is to connect two concentric rows of springs together in series (hence the so called series isolator). This arrangement provides the maximum spring volume for the typical envelope available for the torsional damper.
This arrangement requires the use of one or more plates to serve as a torsional connection between the outer and inner row of springs (the so called “floating flange”). Unfortunately, the inertia of floating flange(s) is substantial for any concentric spring series arrangement. This inertia introduces an addition degree of freedom to the torsional drivetrain system that causes an objectionable vibration in the vehicle. One method of overcoming the additional degree of freedom is to introduce a friction package across one of the concentric spring packages to absorb the energy of the flange mode. However, this approach has the distinct disadvantage of degrading the isolation the damper provides at all frequencies other than the flange mode.
Thus, a problem exists in the field concerning the reduction of vibration produced in the engine that is transmitted through the drive train when a torque converter clutch is engaged with the vehicle engine.